1. Field of the Invention
The present invention relates to a mass spectrometer, a mass filter, a method of mass spectrometry and a method of mass to charge ratio separation.
2. Discussion of the Prior Art
Radio Frequency (RF) ion guides are commonly used for confining and transporting ions. Conventional RF ion guides use an arrangement of electrodes wherein an RF voltage is applied to neighbouring electrodes so that a radial pseudo-potential well or valley is generated in order to radially confine ions within the ion guide. Conventional RF ion guides include quadrupole, hexapole and octapole rod sets. Ion tunnel ion guides are also known which comprise a plurality of stacked rings or electrodes having apertures through which ions are transmitted and wherein opposite phases of an RF voltage supply are applied to adjacent rings.
In addition to ion guides per se, 2D and 3D quadrupole ion traps and quadrupole rod set mass filters are known. Quadrupole rod set mass filters comprise four rod electrodes wherein diametrically opposed rods are maintained at the same AC and DC potential. Adjacent or neighbouring rods are supplied with opposite phases of an AC voltage supply. A DC potential difference is maintained between adjacent rods when the set is operated in a mass filtering mode. Ions having specific mass to charge ratios are arranged to pass through the quadrupole rod set mass filter with substantially stable trajectories. However, all other ions are arranged so as to have substantially unstable trajectories as they pass through the quadrupole rod set mass filter. Those ions which have unstable trajectories are not radially confined within the quadrupole mass filter and will therefore, most likely, hit one of the rods and be lost. Conventional quadrupole rod set mass filters therefore suffer from the problem that although they may transmit specific ions having normally a relatively narrow or specific range of mass to charge ratios with a high transmission efficiency, all other ions will be lost. Furthermore, conventional quadrupole rod set mass filters are also normally relatively long and this makes the miniaturisation of mass spectrometers problematic.
It is therefore desired to provide an improved mass filter for use in a mass spectrometer.